For many years, the primary object of a storm sewer system in urban and suburban developments has been to collect runoff water in catch basins, etc., duct the water from catch basins into a storm sewer pipe and provide suitable ducting to lead the captured water to a suitable dumping site such as a lake, river or ocean. Little thought has been given to returning some or all of the runoff water to the soil through which the storm sewer system passes.
Because of increasing public awareness of the gradual depletion of the underwater aquifer, more and more attention has been given to the conservation and replenishment of the underground aquifer.
To this end, extensive studies have been done to determine possible methods of restoring the aquifers in areas of urban development which are serviced by storm sewer systems which, until recently, have functioned in the classical manner of ducting all the runoff water into some type of sink.
Such a study was carried out in the city of Etobicoke and is reported in publication ISBN 0-7778-72854 entitled Post-Construction Evaluation of Stormwater Exfiltration and Filtration Systems by A. M. Candaras Associates Inc. (Project No. 9321) under publication PIBS 3622E (Queens Printer for Ontario 1997).
This study details methods of controlling runoff and the introduction of storm sewage effluent into the soil surrounding a storm sewer installation. In the system studied, a conventional storm sewer system is augmented by connecting a parallel drainage system comprising a system of perforated drainage pipes located in a sewer trench just below the standard storm sewer pipe used (previously) to carry the total runoff water. The sewer trench, in which the perforated pipes are located, is purposely filled with stone aggregate. The perforated pipes and the storm sewer pipe are connected into a conventional manhole system, so that the storm sewer pipe is located in the sewer trench a predetermined distance above the perforated drainage pipes. The perforated pipes are plugged at each connected downstream manhole to prevent the passage of the runoff water contained in the perforated pipes into the downstream manhole and to encourage the water in the perforated pipes to flow into the stone aggregate surrounding the perforated pipes.
At times of substantial runoff events, it will be found that the perforated pipe system may not be able to contend with the entire runoff and as the water in the manholes builds up to the storm sewer outlets, the standard storm sewer begins to conduct the excess runoff to the usual sink.